Japanese encephalitis (JE) virus is the most important cause of encephalitis in children in Asia based on its frequency and severity. It is a serious disease with a case fatality rate of up to 30% and sequelae in a further 30-50% of children. JE is an endemic disease in Cambodia, with surveillance demonstrating JE causes at least 20% of all cases of eningoencephalitis in children fewer than 15 years of age. Although there is no curative treatment for JE, the disease is vaccine-preventable. A safe, effective, affordable JE vaccine, the live, attenuated SA 14-14-2 JE vaccine, is increasingly becoming available in Asia. The aim of this work is to demonstrate the impact of a JE immunization campaign utilizing this vaccine. Population-based meningoencephalitis surveillance in one province in Cambodia will be strengthened to ensure good data are available on disease rates before immunization. Following collection of at least one year of surveillance data, a JE vaccination campaign for at risk children will be implemented in this province. The campaign will target children under 15 years of age, the age group at highest risk of disease. Surveillance data gathered for at least one year after the campaign will demonstrate the proportion of meningoencephalitis prevented by JE vaccine. Rates of meningoencephalitis over the same period will also be monitored in a neighboring province without vaccination. The project activities will demonstrate the true impact on rates of meningoencephalitis and JE of a mass JE immunization campaign with SA 14-14-2 JE vaccine. This information will be useful within Cambodia, for other JE-endemic countries considering vaccine introduction, and for organizations providing technical support or resources to improve control of JE in Asia. These organizations include the World Health Organization (WHO), UNICEF, The Global Alliance for Vaccination and Immunization (GAVI), and other government and non-government organizations.